Colour picture tube with improved color purity

ABSTRACT

In a colour picture tube of the type comprising an evacuated envelope and a fluorescent screen including a plurality of phosphor units and a coating of a black non-luminous substance applied onto the areas between the phosphor units, the phosphor units are provided over substantially the entire surface of the fluorescent screen at a substantially equal pitch, and each of the phosphor units has an area larger than the cross-sectional area of an electron beam impinging upon the fluorescent screen, and the area of the phosphor units at the peripheral portion is larger than or substantially equal to that of the units at the central portion.

United States Patent [1 1 [111 3,835,347 [451 Sept. 10, 1974 Kauai 1 COLOUR PICTURE TUBE WITH IMPROVED COLOR PURITY [75] Inventor: Hiromi Kanai, Mobara City, Japan [73] Assignee: Hitachi, Ltd., Tokyo, Japan [22] Filed: July 21, 1972 [21] Appl. No.: 273,822

[30] Foreign Application Priority Data Aug. 11, 1971 Japan 46-60312 [52] US. Cl. 313/92 R, 313/69 C [51] Int. Cl. HOlj 29/32 [58] Field of Search .313/92 R, 92 B, 69 C [56] References Cited UNITED STATES PATENTS 3,146,368 8/1964 Flore et al 313/92 B 3,344,301 9/1967 Kaplan 313/92 B 3,614,504 lO/1971 Kaplan 313/92 B Primary Examiner-Herman Karl Saalbach Assistant ExaminerSiegfried H. Grimm [5 7] ABSTRACT In a colour picture tube of the type comprising an evacuated envelope and a fluorescent screen including a plurality of phosphor units and a coating of a black non-luminous substance applied onto the areas between the phosphor units, the phosphor units are provided over substantially the entire surface of the fluorescent screen at a substantially equal pitch, and each of the phosphor units has an area larger than the cross-sectional area of an electron beam impinging upon the fluorescent screen, and the area of the phosphor units at the peripheral portion is larger than or substantially equal to that of the units at the central portion.

2 Claims, 2 Drawing Figures PAIENIEUsmmsn Essie ,--PRIORART DISTANBE FROM CENTER SCREEN 2:555 5c E E;

COLOUR PICTURE TUBE WITH IMPROVED COLOR PURITY BACKGROUND OF THE INVENTION This invention relates to a colour picture tube, more particularly to a colour picture tube having a fluorescent surface having a plurality of phosphor dot units each having an area larger than that of an electron beam impinging upon the fluorescent surface and wherein the spaces surrounding the phosphor dot units are covered by a black nonluminous substance.

Shadow mask type colour picture tubes are generally used. In the colour picture tube of this type, an electron beam emanated from an electron gun structure is caused to impinge upon a selected dot, thus causing the same to luminescence.

Generally, the diameter of the dot is larger than that of the electron beam for the purpose of preventing the electron beam from becoming out of alignment with a selected dot due to errors involved in the fabrication of the dots or the electron gun. The difference in the diameters of the dot and electron beam is termed a guard ring and it is known that the adjustment of colour purity becomes easy as the size of the guard ring is increased. The phenomenon in which the electron beam excites the dots other than those selected thus emanating different colours is called mislanding. Most of, the mislanding is caused by the terrestrial magnetism or by the distortion of the deflection magnetic field. distortion of the deflection field also causes an adverse arrangement of the beam trio near the periphery of the picture. For this reason, in the conventional shadow mask type colour picture tube, it is necessary to decrease the size of the phosphor dots towards the periph-.

ery from the center of the fluorescent screen. Further, the mislanding occurs more frequently near the periphery than at the center of the picture.

To eliminate the phenomenon of mislanding it has been proposed to decrease by 0.04't0 0.09 mm the diameter of the perforations through the shadow mask at the periphery than at the central portion, thus increasing the area of the guard ring.

Recently, however, it is common to use an improved colour picture tube called the matrix type wherein the areas between the phosphor dots on the fluorescent screen are coated by a black nonluminescent substance so as to decrease light reflection from the areas about the dots thereby producing an image of clearly defined contrast under any circumstances. In such a matrix type colour picture tube, the diameter of the phosphor dots is made smaller about 40 to 60 microns than in the conventional colour picture tube for the purpose of increasing the light absortion area afforded by the black nonluminous substance. There is a limit for increasing the area of the guard ring by the above described measure for eliminating the phenomenon of mislanding so that adjustment of the colour purity is extremely difficult. Following table shows the diameters of the phosphor dots of the prior art matrix type colour picture tubes having an effective diameter of 20 inches and the pitches of the small perforations of 0.70 and 0.56 mm, respectively.

TABLE Position on the fluorescent screen Pitch of the perforation (mm) As can be clearly noted from this table the diameter of the dots is about microns larger at the central portion than in the peripheral portion in each of the matrix type colour picture tubes having different pitches.

Ina conventional colourpicture tube not using a coating of the black nonluminous substance between the phosphor dots the diameter of the dots at the peripheral portion is designed to be about 0.32 mm where the pitch of the fine perforations of the shadow mask equals 0.56 mm, for example.

As above described, in the matrix type colour picture tube, thewidth of the guard ring is about 20 to 30 microns smaller than that of the conventional colour picture tube and there is a limit for increasing the width of the guard ring for the purpose of eliminating mislanding. For this reason, adjustment of the colour purity has been difficult. Further, in the colour picture tube of the type other than the matrix type it has been proposed to increase the pitch of the fine perforations of the shadow mask at the peripheral portion thereof while at the same time increasing the diameter of the phosphor dots. However, increase in the pitch of the fine perforations at the peripheral portion renders substantially impossible to arrange the perforations and dots in regular triangles. Even if possible, a colour picture tube of such construction has an extremely poor colour purity so that it can not be used practically. I

SUMMARY OF THE INVENTION Accordingly, it is an object of this invention to provide an improved colour picture tube capable of forming picture images of excellent colour purity.

A further object of this invention is to provide a colour picture tube in which the adjustment of the colour purity can be made readily.

Still further object of this invention is to provide an improved colour picture tube capable of forming picture images of an excellent contrast.

Another object of this invention is to provide a colour picture tube suitable for mass production.

In accordance with this invention there is provided a colour picture tube of the type comprising an evacuated envelope including a neck, a funnel and a face plate, a fluorescent screen formed on the inner surface of the face plate, the fluorescent screen having a plurality of phosphor units and a coating of a black nonluminous substance applied onto the areas between the phosphor units, an electron gun structure disposed in the neck, and a colour selection electrode disposed close to the fluorescent screen between the same and the electron gun structure, characterized in that the phosphor units areprovided over substantially the entire inner surface of the face plate at a substantially equal pitch, and that each of the phosphor units has an area larger than the cross-sectional area of an electron beam emanated from the electron gun structure and impinging upon the fluorescent screen.

I The phosphor units have substantially the same size over the entire surface of the fluorescent screen or have larger size at the peripheral portion of the fluoresent screen than at the central portion thereof.

BRIEF DESCRIPTION OF THE DRAWING In the accompanying drawing:

FIG. 1 is a longitudinal sectional view of a colour picture tube showing the basic construction of the invention and FIG. 2 is a graph showing the relationship between the position of the phosphor dot unit and the diameter of the dots.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 shows a basic construction of a colour picture tube to which the invention can be applied. As shown, the tube comprises an evacuated glass envelope 1 constituted by a neck 2 and a face plate 4, an electron gun structure 3 disposed in the neck 2, a fluorescent screen formed on the inner surface of the face plate 4 and provided with a plurality of trios of phosphor dots (not shown) for red, green and blue, and a shadow mask or colour selection electrode 6 disposed at a predetermined distance from the fluorescent screen 5. Although not shown in the drawing, the shadow mask 6 is provided with a plurality of fine perforations corresponding to the trios of phosphor dots for transmitting an electron beam.

In the colour picture tube of this construction, the electron beam emanated from the electron gun structure 3 is caused to impinge upon the desired dots on the fluorescent screen 5 through a corresponding perforation of shadow mask 6 thereby causing the dots to lumineseence.

In accordance with this invention, the phosphor dot units formed on the entire surface of the face plate are arranged with substantially equal pitches, or spacing and are proportioned such thatthey will have wider area than that of the cross-sectional area of the electron beam impinging thereon and that the area of the phosphor dot units at the peripheral portion is larger than or substantially equal to that of the dot units at the central portion. Further, a coating of a nonluminous substance is applied onto the fluorescent screen at portions between dot units.

More particularly, the diameter of the phosphor dot is proportioned to gradually increase from the center toward the peripheral portion or not to vary substantially and the areas between adjacent dots are coated with a black nonluminous substance. The pitch of the dots is substantially equal over the entire surface of the fluorescent screen. Moreover, the area of each dot is made larger than the cross-sectional area of the electron beam impinging thereon.

FIG. 2 shows a graph wherein the abscissa represents the distance from the center of the fluorescent screen of a colour picture tube embodying the invention, whereas the ordinate shows the dot diameter variation at the fluorescent screen. In this figure, curve 9 shows a case wherein the diameter of the dots at the peripheral portion of the fluorescent screen is larger than that of the dots at the central portion, whereas curve 10 a case wherein the dots at the central portion and the peripheral portion have the same diameter. The prior art variation previously mentioned is also shown.

For example, where the colour picture tube has an effective diameter of inches and a pitch of the perforations of 0.56 mm, the diameter of the dots is about 0.31 mm at the central portion and larger than about 0.31 mm at the peripheral portion. Consequently, according to this invention the diameter of the guard ring is larger at the peripheral than that of the conventional design by about 10 microns so that the invention provides a matrix type colour picture tube in which the ad- 15 justment of the colour purity can be made very easily.

In the prior art colour picture tube, the percentage of the area of the coating of the black nonluminous substance applied on the areas between dots amounts to about 30 percent at the central portion and to about 50 percent at the peripheral portion, but in the colour picture tube embodying the invention this percentage amounts to about 30 percent at the central portion but less than about 30 percent at the peripheral portion. Since the nonluminous area at the central portion is substantially the same as in the prior design, it is possible to substantially increase contrast.

Although in the above embodiment, the invention has been described in terms of a shadow mask type colour picture tube provided with phosphor dots, it should be understood that the invention can also be applied to a grid mask type colour picture tube having phosphor units in the form of stripes.

I claim:

1. A colour picture tube of the type comprising an evacuated envelope including a neck, a funnel and a face plate, a fluorescent screen formed on the inner surface of said face plate, said fluorescent screen having a plurality of phosphor units comprising a trio of phosphor dots for different colors and a coating of a black nonluminous substance applied onto the areas between said phosphor dots, an electron gun structure disposed in said neck, and a colour selection electrode disposed close to said fluorescent screen between the same and said electron gun structure, characterized in that said phosphor dots are provided over substantially the entire inner surface of said face plate at a substantially equal pitch, that each of the phosphor dots has an area larger than the cross-sectional area of an electron beam emanated from said electron gun structure and impinging upon said fluorescent screen, and that the size of said phosphor dots is given by the following equation:

where A=size of phosphor dot at the peripheral portion, B=size of phosphor dot at the central portion.

2. The colour picture tube according to claim 1 wherein said colour selection electrodes comprises a shadow mask provided with a plurality of fine perforations corresponding to said trios of phosphor dots. 

1. A colour picture tube of the type comprising an evacuated envelope including a neck, a funnel and a face plate, a fluorescent screen formed on the inner surface of said face plate, said fluorescent screen having a plurality of phosphor units comprising a trio of phosphor dots for different colors and a coating of a black nonluminous substance applied onto the areas between said phosphor dots, an electron gun structure disposed in said neck, and a colour selection electrode disposed close to said fluorescent screen between the same and said electron gun structure, characterized in that said phosphor dots are provided over substantially the entire inner surface of said face plate at a substantially equal pitch, that each of the phosphor dots has an area larger than the cross-sectional area of an electron beam emanated from said electron gun structure and impinging upon said fluorescent screen, and that the size of said phosphor dots is given by the following equation: A > B where A size of phosphor dot at the peripheral portion, B size of phosphor dot at the central portion.
 2. The colour picture tube according to claim 1 wherein said colour selection electrodes comprises a shadow mask provided with a plurality of fine perforations corresponding to said trios of phosphor dots. 